ASTM F1404-92(1999)

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Designation:F 1404–92 (Reapproved 1999)
Test Method for
Crystallographic Perfection of Gallium Arsenide by Molten
Potassium Hydroxide (KOH) Etch Technique
This standard is issued under the fixed designation F 1404; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope by Preferential Etch Techniques
1.1 This test method is used to determine whether an ingot
3. Summary of Test Method
or wafer of gallium arsenide is monocrystalline and, if so, to
3.1 The determination of the etch pit density is only
measure the etch pit density and to judge the nature of crystal
meaningfulformonocrystallinematerial.Afteramechanicalor
imperfections. To the extent possible, it follows the corre-
chemical polish, or both, of the sample surface, the sample is
sponding test method for silicon, Test Method F 47. Test
etched in molten KOH. This agent preferentially attacks the
Method F 47 also presents the definition of many crystallo-
gallium arsenide surface in regions of crystal imperfections,
graphic terms, applicable to this test method.
suchaslowanglegrainboundaries,twinlamellae,precipitates,
1.2 This procedure is suitable for gallium arsenide crystals
slip lines, and dislocations. The etched surface is examined
with etch pit densities between 0 and 200 000/cm .
microscopically to characterize these imperfections, and deter-
1.3 Gallium arsenide, either doped or undoped, and with
mine their density.
various electrical properties, may be evaluated by this test
3.2 Viewed through an optical microscope, etch pits appear
method. The front surface normal direction of the sample must
as dark elongated hexagonal pits.The etch pit density (EPD) is
be parallel to the <001> within 6 5° and must be suitably
determinedbycountingthesepitsatninedifferentstandardized
prepared by polishing or etching, or both. Unremoved process-
locations across the sample along <011> and <001> directions.
ing damage may lead to etch pits, obscuring the quality of the
Alens micrometer or a grid installed in the microscope is used
bulk crystal.
to define the sampling area. The reported EPD is obtained by
1.4 This standard does not purport to address all of the
averaging the EPD values in the nine counted areas.
safety problems, if any, associated with its use. It is the
3.2.1 The orientation of the elongated KOH etch pits may
responsibility of the user of this standard to establish appro-
also be used to determine the crystal orientation prior to the
priate safety and health practices and to determine the
addition of flats to gallium arsenide (GaAs) wafers or crystals.
applicability of regulatory limitations prior to use. Specific
hazard statements are given in Section 8.
4. Significance and Use
4.1 The use of GaAs for semiconductor devices requires a
2. Referenced Documents
2 consistent atomic lattice structure. However, lattice or crystal
2.1 ASTM Standards:
line defects of various types and quantities are always present,
D 1125 Test Methods for Electrical Conductivity and Re-
and rarely homogeneously distributed. It is important to
sistivity of Water
determine the mean value and the spatial distribution of the
E 177 Practice for Use of the Terms Precision and Bias in
etch pit density.
ASTM Test Methods
F 26 Test Methods for Determining the Orientation of a
5. Characteristics of Revealed Imperfections
Semiconductive Single Crystal
5.1 The KOH etch of the specimen surface reveals patterns
F 47 Test Method for Crystallographic Perfection of Silicon
that are characteristic for several of the crystalline defects
described in detail in Test Method F 47.
5.1.1 Dislocations on {100} GaAs surfaces are character-
This test method is under the jurisdiction of F-1 on Electronics and is the direct ized by microscopic anisotropic six-sided etch pits. The size of
responsibility of Subcommittee F01.15 on Gallium Arsenide.
the pits depends on the consistency of the etch and the etching
Current edition approved May 15, 1992. Published July 1992.
time and will be typically 25 to 50 µm for the procedure
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. Discontinued; see 1997 Annual Book of ASTM Standards, Vol 10.05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F 1404–92 (1999)
described in Section 9. Because the sides of these pits are not 7.2 Purity of Water— Reference to water shall be under-
normal to the incident light, they appear dark under vertical stood to mean either distilled or deionized water with a
field illumination. The use of a Nomarski microscope is resistivity greater than 2 MV·cm at 25°C, as determined by the
optional. nonreferee method of Test Methods D 1125.
5.1.2 Lineage, a precursor to a low-angle boundary, appears 7.3 Chemical Polish— One of the following:
as a linear array of etch pits with a density greater than 25 7.3.1 PolishingEtch,(suchasbromine/methanol,orsulfuric
pits/mm. For this test method, linear arrays less than 0.5 mm in acid/hydrogen peroxide).
length are not considered lineage. The individual etch pits are 7.3.2 Sodium Hypochlorite.
aligned end to end, or side to side. The lineage does not 7.4 Lapping Abrasive— Alumina, Size 5 (0.06 to 0.3 µm).
necessarily follow a <110> direction. 7.5 Degreasing Chemicals—As required according to pre-
5.1.3 Slip is evidenced by a pattern of one or more straight vious process such as:
lines of etch pits that do not necessarily touch each other. The 7.5.1 1,1,1–trichloroethane (TCA 1-1-1),
endsoftheanisotropicetchpitswillbeonacommonline.This 7.5.2 Acetone,
line of etch pits will be in a <110> direction. 7.5.3 Isopropanol (2-propanol), and
5.1.4 A grain boundary appears as a grooved line of any 7.5.4 Other Wax-Removing Solvent.
length in which individual etch pits cannot be resolved 7.6 Defect Etch:
microscopicallyat2003magnification.Thegroovedlinesmay 7.6.1 Potassium Hydroxide (KOH), anhydrous.
enclose an area of the etched surface or extend to the periphery
of the specimen. 8. Hazards
5.1.5 A twin boundary appears as a straight line at the
8.1 The chemicals used in this evaluation procedure are
intersection of a crystallographic plane (usually a <111> plane)
potentially harmful and must be handled with the utmost care
and the etched surface under examination. Two parallel twin
at all times. Read the most current copy of the Material Safety
boundaries that are separated by only a few crystal lattice
Data Sheet (MSDS) for each chemical used. Wear protective
planes form a twin lamella that appears as a straight grooved
gloves and a safety mask so that molten KOH cannot contact
line.
your skin. Safety glasses must be worn at all times. Observe
common laboratory safety precautions. Dispose of all chemi-
6. Apparatus
cals properly.
6.1 Slicing Equipment—Typically an inside diameter (ID)
saw. Such a saw produces a minimum amount of cutting
9. Sample
damage.
9.1 The wafer to be measured must be free of inclusions,
6.2 Wafer Preparation Equipment—This equipment in-
large grains and twins. Those would interfere with the deter-
cludes lapping and polishing facilities capable of removing a
mination of the average EPD value.
minimum of 12µ m from the surface to be characterized. A
9.2 The procedure applies to crystals grown by any method,
polishing etch may be used in place of the wafer polisher, but
such as Liquid Encapsulated Czochralski (LEC), Horizontal
will require substantially more stock removal (50 µm mini-
Bridgman (HB), and Vertical Gradient Freeze (VGF). The
mum).
sample surface must be oriented within 5° parallel to a <100>
6.3 Laboratory Equipment—Nickel crucibles and tweezers
plane.
are necessary to work with molten KOH. Platinum or zirco-
nium have also been used successfully and can be substituted
10. Procedure
for the nickel tools.
10.1 Orient the ingot so that the front surface normal
6.4 Device, capable of heating the crucible with the samples
direction of the sample is parallel to the <001> within 5°.
to 500°C.
Either the X-ray or the optical method of Test Methods F 26
6.5 Microscope, provided with 103 and 203 magnification
can be applied. Cut a wafer at least 0.025 in.-thick from the
objective lenses, a 103 magnification eye piece, a 0.5-mm
crystal. If the crystal has no flats, notch a {110} edge of the
pitch micrometer, and a metric stage micrometer.
wafer. This will later permit locating areas for etch pit
7. Reagents and Materials
counting.LECcrystalsgrownon<100>resultinroundwafers.
HB wafers are D-shaped, unless processed into round wafers.
7.1 Purity of Reagents—Reagent grade chemicals shall be
10.2 Polish the wafer. Afterwards, the wafer must be
used in all tests. Where available, all reagents shall conform to
cleaned and dried. Make sure that a minimum of 0.0015 in. has
the specifications of the Committee on Analytical Reagents of
been removed from each side. If the wafer appears contami-
the American Chemical Society. Other grades may be used,
nated or not fully polished, repeat the polishing process.
provided it is first ascertained that the reagent is of sufficiently
10.3 If the wafer was exposed to wax during previous
high purity that it will not reduce the accuracy of the test.
processes, it must be fully degreased. Immerse the wafer for
five min in hot (60°C) 1,1,1–trichloroethane, followed by 5
“Reagent Chemicals,American Chemical Society Specifications,”Am. Chemi-
min in cold 1,1,1–trichloroethane, followed by an acetone dip
cal Soc., Washington, DC. For suggestions on the testing of reagents not listed by
and by an isopropanol dip. Finally, immerse the wafer for five
theAmerican Chemical Society, see “Reagent Chemicals and Standards,” by Joseph
min in hot (60°C) isopropanol; remove the wafer and allow to
Rosin, D. Van Nostrand Co., Inc., New York, NY, and the “United States
Pharmacopeia.” air dry.
F 1404–92 (1999)
10.4 Place the wafer in the center of the bottom of a nickel 10.14.2 For specimens with EPD greater than 500/cm ,a
crucible. If several wafers are treated simultaneously, the 103 objective lens is recommended. A203 objective is rec-
wafers should not touch each other or the walls of the crucible. ommended when counting EPDs in the range of 30 000 to
A large, flat crucible may be necessary. 200 000/cm .
10.5 Cover the wafers with KOH completely. Use KOH 10.15 Round LEC Wafers— To count the EPD, place the
sparingly and avoid skin contact; remember to wear eye etched specimen on the microscope stage so that the major flat
protection. faces towards the operator. If the sample has no flats, orient it
10.6 Preheat the heating device to 450°C. Place the lid on so that the long axis of the pits point toward the operator. Note
the crucible. Place the crucible on the heating device. Check the location of the reference notch from 10.1 for test records.
after 3 min to verify that KOH is completely molten; if not, 10.15.1 Measure the diameter of the wafer using theVernier
increase the heat-up time. Leave the crucible on the heating scale of the microscope stage. Determine the nine counting
device for additional 7 min. positions according to Table 1 and Fig. 1.
10.7 Usinglongtongs,removethecruciblefromtheheating
NOTE 1—The order of the counting locations differs from Test Method
device and place it on a hot pad nearby. Remove the lid.
F 47 to avoid interference with the flats on GaAs wafers.
10.8 Pour the molten KOH into a second nickel crucible.
10.15.2 With the wafer flat facing the operator, move the
The molten KOH can be used once more for a second batch of
wafer so that the 0.5-mm micrometre disk is centered in
wafers.
Position 1. Count the etch pits and record the results as well as
10.9 Using the nickel tweezers, place the wafer(s) nearly
the microscope objective magnification. Repeat the procedure
uprightalongthewallofthecruciblesothatmostofthemolten
for Position 2 etc. Upon reaching Position 5, rotate the wafer
KOH drips off. This will also allow for easy wafer removal
45° clockwise and continue for Positions 6 through 9. In
once the KOH freezes. Allow the wafers to cool for 5 min.
calculating the average EPD, be sure to count Position 3 only
10.10 Place the wafer(s) under running dionized water until
once.
the remaining solid KOH is fully removed.Any KOH remain-
10.16 {100} Oriented D-Shaped Wafers From Boules
ing in the crucible may be removed the same way.
Grown by the Bridgman Method:
10.11 Examine the wafer. The previously polished surface
10.16.1 The etch pit density of these wafers will be counted
should now have a dull, matte appearance. It may exhibit some
at9locationsthataredifferentfromthelocationsusedforLEC
cellular structures. Examine it under the microscope with a
wafers. Because of the wafer shape asymmetry, and because
103 objective lens to determine if the proper development of
the KOH etch pits on D-shaped wafers have a more uniform
etch pits has occurred. If no pits have developed, repeat 10.4
distribution, the two counting axes are chosen at 90° to one
through 10.11 with an adjustment of the heating period, for
another as shown in Fig. 2.
example, to a total time of 15 min.
10.16.2 Tocounttheetchpits,constructthetwoaxesofFig.
10.12 Adjustments for Overetched Wafers:
2. The first axis is perpendicular to the flat part of the wafer.
10.12.1 Iflargeandovercrowdedpitsarepresentthesample
The second axis is perpendicular to the first axis, and bisects it.
may have been etched too long, or at too high a temperature.
10.12.2 Changetheobjectivelensto203magnificationand 10.16.3 Measure the length of each axis to the nearest
millimetre.
check again. If the pits are still too difficult to count, repeat
10.1 to 10.11 for a shorter etch time, for example, to a total 10.16.4 Locate the nine counting positions along both axes
according to Table 2. With the flat side of the wafer pointing
time of 7 min.
10.13 Foretchpitdensitieslessthan500/cm ,chooseafield away from the operator, begin with Location 1 and continue to
Position 5. Rotate the wafer 90° counterclockwise and proceed
of view that results in a minimum of 20 pits and a maximum
of 150 pits in each counting area. The field selection should with Positions 6 through 9. Record results and conditions as
described in 10.15.2.
remain as representative as possible.
10.14 Theetchpitdensityistobeestablishedin9locations. 10.17 Very High and Ver
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